1. Field
Embodiments of the present invention relate to a multielectrode for recording low amplitude signals originating from bioelectrical potential differences, to a method of processing signals recorded by the invented multielectrode, to a system for recording and amplifying low amplitude bioelectrical signals, whereby an improved signal-to-noise ratio can be achieved, and to a process for manufacturing the invented multielectrode.
2. Background Art
In examinations recording bioelectrical signals, such as in ECG (electrocardiography), EMG (electromyography) and ENeG (electroneurography), the bioelectrical signals are detected and recorded by electrodes. One recording electrode, especially used in ENeG, comprises e.g. two large chlorinated silver plates or two half spherical metal surfaces, e.g. of silver, applied to a patient, in the vicinity of a nerve. The size and shape of the two electrically conducting surfaces of the electrode depend on the individual application and design, the distance between them is normally fixed, e.g. to 20-30 mm, and they may be enclosed in a plastic mould. Pieces of felt material soaked in saline or some other electrically conducting liquid are positioned in the recesses holding the electrode surfaces in order to establish contact between the electrode surfaces and the skin.
The electrically conducting surfaces constituting the electrode may also be mounted individually, directly on the skin in appropriate individual positions by using adhesive tape. When recording small amplitude signals from a peripheral limb nerve, the electrodes are positioned and fixed to the skin overlying the nerve, for example by adhesive tape or a Velcro strap attached around the electrode and the limb. The recording electrode is preferably attached to the skin with the two electrically conducting recording surfaces positioned directly above and along the nerve, minimizing the distance between the recording surfaces and the nerve.
A very high amplification is necessary in the recording system, since the amplitude of the neural signals derived from normal human limb nerves is low, between 100 and 5 microvolt. By superimposing repeated responses or by using an averaging procedure, an improvement of the signal-to-noise ratio of successively recorded nerve responses can be achieved, such that the limit for discrimination of reliable responses is around 1 microvolt.
However, there are several drawbacks with these electrodes. Due to the low amplitude of the nerve signals, the accuracy of the recording is easily disturbed. The recording procedure may have to be repeated when other simultaneously recorded potentials interfere due to e.g. sweating and movements of the patient, or when concurrent 50 Hz-disturbances occur. Since an averaging procedure is utilized, the intermittent electrical stimulation used to induce the neural activity can be prolonged, thereby causing further discomfort to the patient.
Another available technique uses near nerve recording by needle macroelectrodes. A needle macroelectrode is a needle electrode with a relatively large recording area at the tip, which is inserted percutaneously (through the skin) and brought close to or in outer contact with the nerve. A reference electrode is positioned subcutaneously nearby. Since the needle tip is located close to the activated nerve fibres in near nerve recording, the signal-to-noise-ratio is improved. In combination with averaging procedures, discrimination of signals with an amplitude of only 0.5-0.2 microvolt is possible.
In microneurography, which is another recording technique, a solid tungsten microelectrode or a concentric electrode with an outer diameter of only 200 micrometers is inserted percutaneously and positioned intraneurally. The very small surface of the active recording electrode is brought in intimate contact with nerve fibres within an individual nerve fascicle, while the reference electrode surface is positioned nearby, thereby permitting the recording of an electroneurogram of electrically induced nerve responses derived from the entire nerve fibre spectrum, i.e. from both thick and thin myelinated fibres and from thin, unmyelinated fibres, having diameters between 20-1 micrometers and conduction velocities between 70-1 msec. This is the only technique in man that also allows recording from single myelinated and unmyelinated nerve fibres in response to various natural stimuli applied within the innervation area of the impaled fascicle.
However, these procedures, using sterilized needle electrodes, are technically very demanding, time consuming and manually difficult to execute. They are, therefore, unsuitable as clinically routine diagnostic tools.
Related art is also described e.g. in U.S. Pat. No. 5,976,094.
The closest prior art is revealed in U.S. Pat. No. 5,660,177, disclosing a bioelectrical sensing electrode comprising an array of electrodes, by which the DC-potential can be recorded at several different detection sites on a patient, in order to screen e.g. a breast, (see e.g. FIG. 1 and column 5, lines 32-60). Prior art is also disclosed in US 2003/009096, which describes a sensor system for measuring bioelectrical potentials on different detection sites on the head of a patient, by using an array of three electrodes. In all techniques described in these prior art documents, the biological signal of interest is recorded only one time at the detection site. By contrast, embodiments of this invention provide for an improved recording of bioelectrical potential differences derived from the same bioelectrical impulse generator/s/, e.g. nerve fibre/s/ or muscle fibre/s/ at only one detection site, with improved signal-to-noise ratio achieved by using multiple recordings and a summation of the bioelectrical potential differences derived from said impulse generators at this detection site, using several recording pairs, which are provided on one electrode.
An object of embodiments of this invention is to limit or eliminate some of the described problems when recording low amplitude bioelectric signals and to provide an improved non-invasive recording electrode and a novel procedure to process the recorded signals, whereby in particular the signal-to-noise ratio of the signals is improved compared to prior techniques, making embodiments of the invention suitable for clinical examinations of patients.